For mobile devices, automotive, and other applications, it can be important to have both a high volumetric energy density and a high-rate charge capability for electrochemical batteries and cells. To achieve both goals, one should consider the composition of the electrode materials, the construction of the electrodes, the chemical make-up of the electrolyte, and interfacial aspects of the cells. In other words, from a theoretical perspective, both thermodynamic and kinetic considerations, which are sometimes at odds with each other, should be addressed in order to meet increasingly stringent user demand for high performance electrochemical cells.
For example, there are many competing factors to consider when optimizing a cell's volumetric energy density and rate capability. Volumetric energy density can generally be increased by using electrode materials with high capacity, high redox voltage, and high density and by using electrodes with low porosity, large thickness, and low amounts of inactive components such as binders and conductive additives. Inorganic cathode compounds such as lithium cobalt oxide are generally considered to have high capacity, high redox voltage, and high density. Rate capability, in theory, may be increased by minimizing solid-phase diffusion in favor of surface reactions, e.g., by increasing surface area of anode and/or cathode reactive materials. Such increases in surface areas may alter electrode void volumes.
In addition, electroactive polymeric materials may be used in electrochemical cells. As a general rule, organic radical polymers (ORP), for example, can have high-rate charge capability. However, they also generally have low volumetric energy density. That said, many anion-absorbing polymers are considered to have rapid solid-phase diffusion because of short diffusion path lengths. When such materials are used in one or more electrodes or in an electrolyte, they may theoretically enhance the kinetics associated with enhanced cell rate capabilities.
Rate capability can also be increased by using an electrolyte with a high mobility for the active ion and ensuring that the porosity in the electrode and/or separator materials is sufficient to provide sufficient transport of ions across the cell and avoid the formation of large concentration gradients. It has been theorized that concentration gradients form in all lithium-ion batteries because of transport of the cation from the positive electrode to the negative electrode during charge. If the concentration of an ion or salt in a region of the negative electrode falls to zero, then the reaction of lithium ions with the negative active material cannot proceed in that region. That is, salt depletion may represent a major factor that limits the charge rate capability of electrochemical cells or batteries.
Thus, solutions are needed to avoid exacerbating electrochemical performance problems such as those associated with salt depletion. As discussed below, such solutions may involve, for example: providing electrodes that exhibit a compositional profile such that the anion-absorbing material is present at different volume percentages at the back region relative to the front region of an electrode; using an electrolyte at a sufficiently high enough concentration when the cell is in a fully discharged state; and other techniques that may synergistically operate to enhance charge rate capabilities of electrochemical cells.